The present invention relates to a sensor tube, such as a thermowell, used in measuring a fluid variable in a process. More specifically, the present invention relates to a sensor tube configuration that achieves vortex shedding reduction using a simple manufacturing technique.
Process fluid temperature is an important physical parameter that is often used to control or otherwise monitor a process. A process fluid temperature is typically measured using a temperature sensor, such as a resistance temperature device (RTD), thermocouple or thermistor. The temperature sensor itself is generally not able to withstand direct contact with a process fluid. Thus, a thermally conductive sensor tube, such as a thermowell, is used to interface with the process fluid while protecting the temperature sensor. The process fluid directly contacts the thermowell and heat from the process fluid transfers through the thermowell to the temperature sensor disposed therein. In this manner, the temperature sensor can accurately measure process fluid temperature without directly contacting the process fluid. A thermowell allows replacement of the temperature sensor without having to break the process seal.
Since sensor tubes and thermowells are directly inserted into the process, they are subject to a number of stresses. When thermowells are used in pipes or tanks, they suffer from high fatigue stresses caused by vortex shedding. This vortex shedding occurs at specific frequencies as determined from the Strouhal Number. The Strouhal Number is approximately 0.22 and does vary slightly with Reynolds Number. The Strouhal Number is fsdm/V, where fs is the shedding frequency, dm is the diameter of the cylindrical thermowell and V is the flow stream velocity. When the shedding frequency is close to the natural frequency of the thermowell, the thermowell will violently vibrate at its natural frequency and exceed fatigue stress limits. Generally two velocities are of concern, the largest stresses are caused by crossflow vibration which is the frequency given by the Strouhal Number. There is a second velocity that is of concern is ½ the velocity given by the Strouhal Number. This velocity causes the thermowell to vibrate inline with the flow and is caused by the vortices shed from each side of the thermowell where forces at twice the shedding frequency are generated. This vibration mode usually generates less stress than the crossflow condition, but it still can cause the thermowell to fail in fatigue.
Thermowell designs are usually checked by the requirements of ASME PTC 19.3 TW-2010 and give acceptable flow velocities for the conditions specified. The inline vibration mode is checked for stress levels in vortex frequencies 0.4 to 0.6 of the lowest natural frequency of the thermowell. Some applications in this velocity range will be unacceptable due to fatigue stress levels. This standard requires vortex frequency in all applications to be below 0.8 of the natural frequency.
In some circumstances, vortex shedding forces can lead to breakage of the thermowell due to fatigue stress failure and therefore, loss of pressure containment and potential damage to down stream components due to an unattached part in the pipe.
Some attempts have been made to reduce vortex shedding from thermowells. For example, it is known to attach helical strakes to a thermowell to reduce vortex shedding. United States Patent Publication 2008/0307901 A1 by Jeremy Knight also shows a thermowell or a gas sampling tube with helical strakes attached. Further methods for reducing vortex shedding can be found in a paper by M. M. Zdravkovich entitled, “Review and classification of various aerodynamic and hydrodynamic means for suppressing vortex shedding” Journal of Wind Engineering and Industrial Aerodynamics, 7 (1981) pp. 145-189.
Providing an easily manufacturable sensor tube with effective vortex shedding reduction would represent an important advance to the art of measuring process fluid variables when the process fluid is flowing or otherwise in motion.